Gynecologic ultrasound and obstetric ultrasound are two distinct applications of ultrasound imaging that serve different purposes in the field of women's health. While both involve the use of ultrasound technology to examine the pelvic region, they have different focuses and objectives.

Gynecologic [gynaecologic, Brit.] ultrasound primarily concentrates on the evaluation of the female reproductive organs, including the uterus, ovaries, fallopian tubes, and surrounding structures. It is commonly performed for various gynecological concerns, such as abnormal bleeding, pelvic pain, infertility investigations, and monitoring of reproductive disorders. It can identify signs of inflammation, the presence of free fluid, cysts, and tumors. This non-invasive technique aids in diagnosing and monitoring gynecological pathologies, facilitating early intervention and appropriate treatment. Typically, a transabdominal sonogram is performed with a full bladder to provide an initial assessment. However, if the pelvic ultrasound reveals any abnormalities or fails to provide a clear image of the organs, a more detailed evaluation can be achieved through a transvaginal sonography. This approach allows for improved visualization of the uterus and ovaries by placing the ultrasound probe inside the vagina.

Obstetric ultrasound, also known as prenatal, fetal or pregnancy ultrasound, is the branch of medical imaging that focuses on the use of ultrasound technology to assess the health and development of a fetus during pregnancy. Women with uncomplicated pregnancies commonly undergo an ultrasound examination between the 16th and 20th week of gestation. This routine assessment, performed with a real-time scanner, serves to determine accurate gestational age, monitor fetal size, and assess overall growth. The middle of the pregnancy trimester provides a crucial window for detecting many abnormalities of fetal anatomy. Advanced imaging techniques enable healthcare professionals to identify potential structural issues. Early detection of these abnormalities allows for timely intervention, counseling, and the implementation of appropriate management strategies.
See also Pregnancy Ultrasound, Pelvic Ultrasound, Hysterosalpingo Contrast Sonography and Vaginal Probe.

(OPG) The Oculoplethysmography is used to detect a hemodynamically significant stenosis of the carotid circulation, by measuring the arrival times of pulses in the eye and ear. Indirectly the blood flow in the ophthalmic artery is measured, which branches off the carotid artery and supplies blood to the eye. A delay between pulse arrival in the eye and ear is an indication of a hemodynamically significant stenosis in the vessel whose corresponding pulse is delayed.
Other ultrasound or angiography procedures are necessary to confirm the diagnosis and have also replaced this technique.

See also Plethysmography and Ultrasound Biomicroscopy.

An omnidirectional transducer transmits or receives ultrasound waves in or from any direction with 360 degrees receiving capability.

See also Transmitter, and Receiver.

Opacification is the filling of an echo free region, such as a ventricular cavity, with echoes from microbubble contrast.

The open circuit voltage is the level of the electrical output per one microPascal of acoustic input.